While the Doctor & Romana repair K-9's water damage the population of Tigella, isolated underground by the vicious plant life on the surface, struggle with the dodecahedron that proceeds all their power. The scientific Savants are restricted in what they can do by the religious Deans who worship the Dodecahedron. The leader of the Savants, Zastor, summons the Doctor to help to the consternation of the Deon leader & high priestess Lexa. A group of Gaztaks led by General Grugger kidnap an Earthling taking him to the massive screens on Zolfa Thura where the plant being Meglos outlines it's plan to steal the Dodecahedron. He traps the Tardis in a chronic histeresis(time loop) and assumes the form of the Doctor.

Is this the first story where the Doctor hasn't arrived by the end of the first episode? Quite possibly. A lot of mucking about on Tigella setting the situation up, the bandits plotting with the plant to steal the Dodecahedron and all the time the Doctors & Romana are stuck in the Tardis, and for the last third of the episode they're repeating the same actions.

See the walls of Meglos' command centre? Yup, it's the triangular/hexagonal pattern first seen in The Mutants again. Last seen in Horns of the Nimon, they'll be around a lot for the next few years.

Then we get to the end of the episode and oh look, it's the return of our old favourite, the Doctor's double. See also: The Chase, The Massacre, The Enemy of the World & Android Invasion with companion duplicates also in Inferno, Terror of the Zygons, Android Invasion and Androids of Tara.